1. Field of the Invention
The present invention relates to the field of display techniques, and in particular to an OLED pixel driving circuit and an OLED display device.
2. The Related Arts
The organic light emitting diode (OLED) provides the advantages of active-luminescent, low driving voltage, high emission efficiency, quick response time, high resolution and contrast, near 180° viewing angle, wide operation temperature range, and capability to realize flexible display and large-area full-color display, and is heralded as the most promising display technology.
The OLED is an electroluminescent device driven by electric current; that is, when a current flows through OLED, the OLED illuminates, and the brightness is determined by the current flowing through the OLED. The majority of known integrated circuit (IC) only transmits the voltage signal, and the pixel driving circuit of the OLED display must accomplish the task of translating the voltage signal into a current signal. The conventional pixel driving circuit usually uses a 2T1C structure, i.e., two thin film transistors (TFT) and a capacitor, to translate the voltage into current.
As shown in FIG. 1, a conventional 2T1C pixel driving circuit for driving OLED device comprises: a first TFT T10, second TFT T20, and a capacitor C10. The first TFT T10 is a switching TFT, the second TFT T20 is a driving TFT, and the capacitor T10 is a storage capacitor. Specifically, the first TFT T10 has a gate connected to receive a scan signal Scan, a drain connected to receive a data signal Data, and a source electrically connected to a gate of the second TFT T20 and to one end of the capacitor C10. The second TFT T20 has a drain connected to receive a voltage OVDD of a power source, and a source connected to receive an anode of the OLED D10; the OLED D10 has a cathode connected to receive a common ground voltage OVSS; the capacitor C10 has one end electrically connected to the gate of the second TFT T20, and the other end electrically connected to the source of the second TFT T20. When the OLED displays, the scan signal Scan controls the first TFT T10 to be turned on, the data signal Data passes through the first TFT T10 and enters the gate of the second TFT T20 and the capacitor C10. Then, the first TFT T10 is cut off. Because of the storage of the capacitor C10, the gate voltage of the second TFT T20 stays at the data signal voltage level so that the second TFT T20 stays turned on. The driving current flows through the second TFT T20 to enter the OLED D10 and drives the OLED D10 to emit light.
According to the equation calculating the current flowing through the driving TFT and the OLED:IOLED=K×(Vgs−Vth)2;
Wherein IOLED is the current flowing through the driving TFT and the OLED, K is the intrinsic conductive factor, of the driving TFT, Vgs is the voltage difference across the gate and the source of the driving TFT, and Vth is the threshold voltage of the driving TFT. As seen, the size of IOLED is related to the threshold voltage Vth of the driving TFT.
The above conventional OLED pixel driving circuit is simple in structure, and without a compensation function, and therefore has many shortcomings. The most prominent shortcoming is that the non-uniformity of the TFT fabrication process, the threshold voltages of the driving TFTs of all pixels in the OLED display device will be inconsistent. Moreover, due to long operation time, the ageing of the driving TFT will cause voltage drift of the threshold voltage of the driving TFTs, leading to display unevenness.
The known technologies also include some OLED pixel driving circuit able to sense the threshold voltage of the driving pixel and compensate the threshold voltage to the data signal. However, the voltage of the data signal is affected by the threshold voltage of the OLED. On one hand, the threshold voltage of the OLED must be about 10V, and the light-emitting layer in the OLED must adopt the 3-layered or 4-layered structure. On the other hand, the data signal voltage cannot be higher than the OLED threshold voltage during the data writing stage to ensure that the OLED will not emit light during the data writing stage. Moreover, the gate voltage of the driving TFT is also affected by the OLED threshold voltage and is required to have a voltage level not greater than the sum of the OLED threshold voltage and the driving TFT threshold voltage.